feat: add CSP header (#908)

Co-authored-by: Alessandro (Ale) Segala <43508+ItalyPaleAle@users.noreply.github.com>

backend/internal/service/version_service.go

@@ -0,0 +1,74 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/pocket-id/pocket-id/backend/internal/utils"
+)
+
+const (
+	versionTTL      = 15 * time.Minute
+	versionCheckURL = "https://api.github.com/repos/pocket-id/pocket-id/releases/latest"
+)
+
+type VersionService struct {
+	httpClient *http.Client
+	cache      *utils.Cache[string]
+}
+
+func NewVersionService(httpClient *http.Client) *VersionService {
+	return &VersionService{
+		httpClient: httpClient,
+		cache:      utils.New[string](versionTTL),
+	}
+}
+
+func (s *VersionService) GetLatestVersion(ctx context.Context) (string, error) {
+	version, err := s.cache.GetOrFetch(ctx, func(ctx context.Context) (string, error) {
+		reqCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+		defer cancel()
+
+		req, err := http.NewRequestWithContext(reqCtx, http.MethodGet, versionCheckURL, nil)
+		if err != nil {
+			return "", fmt.Errorf("create GitHub request: %w", err)
+		}
+
+		resp, err := s.httpClient.Do(req)
+		if err != nil {
+			return "", fmt.Errorf("get latest tag: %w", err)
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			return "", fmt.Errorf("GitHub API returned status %d", resp.StatusCode)
+		}
+
+		var payload struct {
+			TagName string `json:"tag_name"`
+		}
+		if err := json.NewDecoder(resp.Body).Decode(&payload); err != nil {
+			return "", fmt.Errorf("decode payload: %w", err)
+		}
+
+		if payload.TagName == "" {
+			return "", fmt.Errorf("GitHub API returned empty tag name")
+		}
+
+		return strings.TrimPrefix(payload.TagName, "v"), nil
+	})
+
+	var staleErr *utils.ErrStale
+	if errors.As(err, &staleErr) {
+		slog.Warn("Failed to fetch latest version, returning stale cache", "error", staleErr.Err)
+		return version, nil
+	}
+
+	return version, err
+}